A3COSMOS: the dust attenuation of star-forming galaxies at z=2.5-4.0 from the COSMOS-ALMA archive

We present an analysis of the dust attenuation of star forming galaxies at z=2.5-4.0 through the relationship between the UV spectral slope ( ß ), stellar mass ( M* ) and the infrared excess (IRX =LIR/LUV ) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A 3 COSMOS team, which includes an unprecedented sample of ~1500 galaxies at z~3 as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 µJy/beam (1 s ). The detection rate is highly mass dependent, decreasing drastically below log(M*/M?)=10.5 . The detected galaxies show that the IRX- ß relationship of massive ( logM*/M?>10 ) main sequence galaxies at z=2.5-4.0 is consistent with that of local galaxies, while starbursts are generally offset by ~0.5dex to larger IRX values. At the low mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX- M* relation at log(M*/M?)>9 exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at logM*/M?<10 . However, our results are consistent with early measurements at z~5.5 , indicating a potential redshift evolution between z~2 and z~6 . Deeper observations targeting low mass galaxies will be required to confirm this finding.